1. Field of the Invention
The present invention relates to a vibrating screen assembly composed of dissimilar materials which is used with a vibrating machine for separating solids and fluids.
2. Prior Art
Vibrating screen assemblies are used on vibrating machines or shakers to separate solid material from liquids and fine solid particles. A frame for the screen assembly may include opposed sides and ends and may include various cross-members and rods. The frame for the screen assembly is removably attached to the vibrating shaker and the screen assembly is vibrated at high frequency by a reciprocating motor.
The dynamic forces brought about by vibration separates the liquid and particles smaller than the pore size of the smallest screen layer. The screen assembly is subject to tremendous stress by virtue of the machinery vibrating it and is subject to stress from the weight of the material to be separated on the top layer of the screen.
While it is possible to replace a screen assembly, frequent replacement entails added cost and down-time for the machinery. Accordingly, extending the life of a screen assembly is highly desirable.
It is advantageous to retain the screen cloth or screen cloths in as tight a condition to the frame as possible. Slackness in the screen cloth or screen cloths may cause undesirable flapping of the screen cloth or cloths. Slackness also may permit the shute strands of the screen cloth to move in relation to the warp strands. This may influence the effectiveness of the screen cloth to filter. It is known that tensioning the screen or screens to the frame will enhance the life of the screen assembly. Indeed, screen tension is believed to be one of the greatest factors to determine the screen life. Accordingly, in some applications, the screen cloth or screen cloths are tensioned prior to being attached and bonded to the frame.
Vibrating screen assemblies may be used to screen materials which contain a heated slurry. In one such application, water is used as a slurry in order to screen calcium carbonate solids. The slurry mixture which is deposited on the top of the screen assembly may be approximately 190.degree. F. and may range up to 250.degree. F.
It is known that as the temperature of solid (and other) materials increases, the average separation of the molecules increases. Accordingly, the materials will expand when heated. The materials that comprise the vibrating screen assembly each have coefficients of thermal expansion. The coefficients of thermal expansion for some common materials are as follows:
CELSIUS SCALE FAHRENHEIT SCALE MATERIAL ##EQU1## ##EQU2## Aluminum 23.9 13.3 Brass 18.7 10.4 Carbon Steel 10.8 6.0 Cast Iron 10.6 5.9 Magnesium 25.2 14.0 Stainless Steel 17.3 9.6 ABS 95 52 Glass-Filled Nylon 81 45 Glass-Filled Poly- 37 21 propylene
Each time the slurry and the materials to be separated are deposited on the screen assembly, heat is transferred to the frame and the screen cloths. The heat transfer causes the screen cloth and the frame to expand. If the frame is caused to expand greater than the screen cloth, the effect will be for the screen cloth or cloths to be tightened.
It is also believed that at the start-up of the screening operation, the heated slurry materials to be screened are deposited on the center of the screen assembly causing the cross-members and rods to be heated quicker than the sides and ends of the frame.
Accordingly, it is a principal object and purpose of the present invention to provide a screen assembly wherein the frame and screen cloth or cloths are composed of different materials and wherein the frame has a greater coefficient of thermal expansion than the screen cloth or screen cloths.
It is a further object and purpose of the present invention to provide a vibrating screen assembly which is self-tensioning when utilized to screen materials above the ambient temperature at which the screen assembly is fabricated.